Herbs & Supplements:
Vitamin K

Supplement Forms / Alternate Names
  • Vitamin K1 (Phylloquinone), Vitamin K2 (Menaquinone), Vitamin K3 (Menadione)
Principal Proposed Uses
  • Treating Medication-Induced Vitamin K Deficiency
Other Proposed Uses
  • Osteoporosis, Menorrhagia (Heavy Menstruation), Nausea

There's a good chance you haven't even heard of vitamin K. However, this obscure member of the vitamin clan is very important for good health. Without it, your blood wouldn't clot properly. There are three forms of vitamin K: K₁ (phylloquinone), found in plants; K₂ (menaquinone), produced by bacteria in your intestines; and K₃ (menadione), a synthetic form.

Vitamin K is used medically to reverse the effects of "blood-thinning" drugs such as warfarin (Coumadin). Growing evidence suggests that it may also be helpful for osteoporosis.

Requirements/Sources

Vitamin K is an essential nutrient, but you need only a tiny amount of it. The official U.S. recommendations for daily intake has been set as follows:

• Infants 0–6 months, 2 mcg
  7–12 months, 2.5 mcg
• Children 1–3 years, 30 mcg
  4–8 years, 55 mcg
• Males 9–13 years, 60 mcg
  14–18 years, 75 mcg
  19 years and older, 120 mcg
• Females 9–13 years, 60 mcg
  14–18 years, 75 mcg
  19 years and older, 90 mcg
• Pregnant women, 90 mcg, preferably the K₁ variety (phylloquinone) (75 mcg if 18 years old or younger)
• Nursing women, 90 mcg, preferably the K₁ variety (75 mcg if 18 years old or younger)

However, a recent study suggests that a higher intake of vitamin K, in the range of 110 mcg daily, might be helpful for preventing osteoporosis in women.¹

Vitamin K (in the form of K₁) is found in green leafy vegetables. Kale, green tea, and turnip greens are the best food sources, providing about 10 times the daily adult requirement in a single serving. Spinach, broccoli, lettuce, and cabbage are very rich sources as well, and you can get perfectly respectable amounts of vitamin K in such common foods as oats, green peas, whole wheat, and green beans, as well as watercress and asparagus.

Vitamin K (in the form of K₂) is also manufactured by bacteria in the intestines, and is a major source of vitamin K. Long-term use of antibiotics can cause a vitamin K deficiency by killing these bacteria. However, this effect seems to be significant only in people who are deficient in vitamin K to begin with.^2–5 Pregnant and postmenopausal women are also sometimes deficient in this vitamin.^6,7,8 In addition, children born to women taking anticonvulsants while pregnant may be significantly deficient in vitamin K, causing them to have bleeding problems and facial bone abnormalities.^9,10,11 Vitamin K supplementation during pregnancy may be helpful for preventing this.

The blood-thinning drug warfarin (Coumadin) works by antagonizing the effects of vitamin K. Conversely, vitamin K supplements, or intake of foods containing high levels of vitamin K, block the action of this medication and can be used as an antidote.¹²

Cephalosporins and possibly other antibiotics may also interfere with vitamin K–dependent blood clotting.^13–16 However, this interaction seems to be significant only in people who have vitamin K–poor diets.

People with disorders of the digestive tract, such as chronic diarrhea, celiac sprue, ulcerative colitis, or Crohn's disease, may become deficient in vitamin K.^17–20 Alcoholism can also lead to vitamin K deficiency.²¹

Therapeutic Dosages

For some purposes, vitamin K has been recommended at a daily dose of 150 to 500 mcg. Although such dosages are much higher than required for nutritional purposes, they are not out of the range of what can be reached through eating plenty of green leafy vegetables.

Therapeutic Uses

Evidence suggests (but does not prove) that vitamin K supplements may be helpful for preventing osteoporosis.^22–32

Based on its ability to help blood clot normally, vitamin K has been proposed as a treatment for excessive menstrual bleeding.³³ However, the last actual study testing this idea was carried out more than 55 years ago.³⁴ Vitamin K has also been recommended for nausea, although there is as yet little evidence that it really works.

What Is the Scientific Evidence for Vitamin K?

Osteoporosis

Vitamin K plays a known biochemical role in the formation of new bone. This has led researchers to look for relationships between vitamin K intake and osteoporosis.

Research has found that people with osteoporosis have much lower blood levels of vitamin K than other people. For example, in a study of 71 postmenopausal women, participants with reduced bone mineral density showed lower serum vitamin K₁ levels than those with normal bone density.³⁵ Similar results have been seen in other studies.^36,37,38

A recent report from 12,700 participants in the Nurses' Health Study found that higher dietary intake of vitamin K is associated with a significantly reduced risk of hip fracture.³⁹

Interestingly, the most common source of vitamin K used by individuals in the study was iceberg lettuce, followed by broccoli, spinach, romaine lettuce, brussels sprouts, and dark greens. Women who ate lettuce each day had only 55% the risk of hip fracture as those who ate it only weekly. However, among women taking estrogen, no benefit was seen, probably because estrogen is so much more powerful.

Another observational study also found evidence that higher vitamin K intake is associated with a reduced incidence of hip fractures.⁴⁰

Research also suggests that supplemental vitamin K can reduce the amount of calcium lost in the urine.^41,42,43 This is indirect evidence of a beneficial effect on bone.

Taken together, these findings suggest that vitamin K supplements might help prevent osteoporosis.

Safety Issues

Vitamin K is probably quite safe at the recommended therapeutic dosages, since those quantities are easily obtained from food.

Vitamin K directly counters the effects of the anticoagulant warfarin. If you are taking warfarin, you should not take vitamin K supplements or alter your dietary intake of vitamin K without doctor supervision.^44,45

Newborns are commonly given vitamin K₁ injections to prevent bleeding problems. Although some have suggested that this practice may increase the risk of cancer,⁴⁶ enormous observational studies have found no such connection (one such trial involved more than a million participants).^47,48

Interactions You Should Know About

If you are taking

• Warfarin (Coumadin): Do not take vitamin K supplements or eat foods high in vitamin K except under the supervision of a physician. (You will need to have your medication dosage adjusted.)
• Cephalosporinsor other antibiotics: You may need more vitamin K if you are already deficient in this nutrient.
• Anticonvulsants—such as phenytoin (Dilantin), carbamazepine, phenobarbital, andprimidone (Mysoline)—and are pregnant: You may need more vitamin K to protect your child.

Last reviewed March 2002 by Medical Review Board

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